Technical Field
The present disclosure relates to an abnormality diagnosis apparatus for a particulate filter that is provided in an exhaust passage of an internal combustion engine to trap PM (particulate matter) included in the exhaust gas.
Description of Background Art
There is a known technique to provide a particulate filter (hereinafter may be simply referred to as “filter”) that is configured to trap PM included in the exhaust gas, in an exhaust passage of an internal combustion engine. A failure such as erosion or breakage may occur in the filter. The occurrence of such a failure increases the amount of PM that is not trapped by the filter but flows out of the filter. The occurrence of such a failure in the filter or the occurrence of an abnormality of the filter, for example, detachment of the filter from the exhaust passage, leads to an increase in PM released to the atmosphere. A proposed technique accordingly provides a PM sensor downstream of the filter in the exhaust passage and diagnoses an abnormality of the filter based on the output value of the PM sensor. A known configuration of the PM sensor used for abnormality diagnosis of the filter has a pair of electrodes as a sensor element and outputs a signal corresponding to the amount of PM depositing between the electrodes.
A technique disclosed in Patent Literature 1 compares an output value of a PM sensor provided downstream of a filter in an exhaust passage with an estimated value of a deposition amount of PM in the PM sensor to determine the presence or the absence of a failure in the filter. The technique described in Patent Literature 1 estimates a flow-out amount of PM from the filter on the assumption that the filter is in a predetermined state, and calculates an estimated value of the deposition amount of PM in the PM sensor based on the integrated value of the estimated flow-out amount of PM. The state of the filter may be detected by comparing the estimated value of the deposition amount of PM with the actual output value of the PM sensor.
A technique disclosed in Patent Literature 2 diagnoses a failure of a filter, based on the time when power supply is started by deposition of PM between electrodes of a PM sensor provided downstream of the filter in an exhaust passage. The technique described in Patent Literature 2 determines that a failure occurs in the filter in the case where the time when power supply is started between the electrodes of the PM sensor is earlier than a power supply start time on the assumption that a failure occurs in the filter.
Patent Literature 3 discloses a technique regarding detection of an abnormality of a PM sensor. The technique described in Patent Literature 3 burns out and removes PM depositing between electrodes of the PM sensor by heating with a heater. An abnormality of the PM sensor is detected, based on a change in resistance value between the electrodes relative to a burning removal time.